Dr. A. Morison1 has shown that in the sheep and pig the atrioventricular bundle is a great avenue for the transmission of nerves from the auricular to the ventricular heart; large and numerous nerve trunks entering the bundle and coursing with it. From these, branches pass off and form plexuses around groups of Purkinje cells, and from these plexuses fine fibrils go to innervate individual cells.

Clinical and experimental evidence go to prove that this bundle conveys the impulse to systolic contraction from the atrial septum to the ventricles.

FIG. 501 Schematic representation of the atrioventricular bundle of His. The bundle, represented in red originates near the orifice of the coronary sinus, undergoes slight enlargement to form a node, passes forward to the ventricular septum, and divides into two limbs. The ultimate distribution cannot be completely shown in this diagram. (See enlarged image)

Vessels and Nerves.The arteries supplying the heart are the right and left coronary from the aorta; the veins end in the right atrium.

The lymphatics end in the thoracic and right lymphatic ducts.

The nerves are derived from the cardiac plexus, which are formed partly from the vagi, and partly from the sympathetic trunks. They are freely distributed both on the surface and in the substance of the heart, the separate nerve filaments being furnished with small ganglia.

The Cardiac Cycle and the Actions of the Valves.By the contractions of the heart the blood is pumped through the arteries to all parts of the body. These contractions occur regularly and at the rate of about seventy per minute. Each wave of contraction or period of activity is followed by a period of rest, the two periods constituting what is known as a cardiac cycle.

Each cardiac cycle consists of three phases, which succeed each other as follows: (1) a short simultaneous contraction of both atria, termed the atrial systole, followed,